Sketchup is an easy-to-learn program for 3D modeling. While it gives you an easy way to design creatively, it is a bit more challening to design accurately. If you are going to make physical models of your designs, you will need to make them small enough to fit on the tool that will make the part. You can send the 3D models to a number of toos, including the ShopBot, CNC Mill and MakerBot, among others.
If you follow these pretty easy steps, you can get yourself and your students into the habit of designing accurately dimensioned objects. Here is a set of screenshots and a sample file that show the steps of this process.